The present invention relates to a process for attenuating the yawing moment in a vehicle having an anti-lock brake system (ABS). The present invention is related to the invention disclosed in commonly assigned patent co-pending application Ser. No. 08/756,593, filed Nov. 27, 1996.
When a vehicle is braked on a roadway with different frictional values on the two sides of the vehicle (.mu. split), the vehicle will tend to pull towards the side of the roadway with the higher frictional value. The driver must counter the yawing moment which causes this tendency to pull by steering against it. This may be a problem for an inexperienced driver, especially if the yawing moment is especially strong due to unfavorable conditions. Unfavorable conditions which exaggerate the yawing moment during braking include a short wheel base, a small load, especially great differences in road condition such as, e.g., concrete on one side of the roadway and ice on the other, and especially strong braking action such as occurs with a regulating anti-lock brake system (ABS).
In vehicles equipped with an anti-lock brake system (ABS), it is already known from DE-OS 28 55 326 (U.S. Pat. No. 4,349,876) to attenuate the yawing moment produced when braking takes place on a road with different surfaces on the right and left sides of the vehicle (.mu. split) by building up the braking pressure in the wheel running with the higher coefficient of friction (high wheel) with a predetermined time delay, and then to limit the braking pressure to a constant value, thereby increasing the stability of the vehicle and relieving the driver of the need to counter the yawing moment. This takes place thanks to special measures taken within the electronic system of the anti-lock brake system (ABS).
Lately, vehicles equipped with so-called electrically controlled braking systems (EBS) have also become known. In such braking systems, e.g., the braking system described in DE-OS 44 06 235, the braking pressure (actual value) in a brake cylinder is adjusted to a desired braking value generated by a braking force transmitter. Normally, these EBS braking systems are also equipped with an anti-lock brake system (ABS). Since pressure sensors are utilized in an electrically controlled braking system (EBS), it is already known to limit the braking pressure difference (.DELTA.P) between the "low wheel" (wheel on side of road with low coefficient of friction) and the "high wheel" (wheel on side of the road with high coefficient of friction), and thereby reduce the yawing moment. See, e.g., DE-OS 24 60 309 (U.S. Pat. No. 3,988,043). Thus, the braking pressure in the high wheel can be controlled by the pressure in the low wheel which is being regulated by the ABS, or else the braking pressure in the high wheel is kept at a constant value and so a mean brake pressure difference (.DELTA.P) is maintained.
It is a disadvantage in these known arrangements that the reduction in yawing moment is constant, i.e., the reduction is not adapted to the different road conditions or the particular vehicle concerned. The maximum braking pressure at the high wheel after the time-delayed build-up depends only on the changes in the low wheel braking pressure as determined by the frictional force of the low wheel. As a result, the maximum braking pressure at the high wheel that would be permitted in maintaining control of vehicle behavior is not always utilized sufficiently. It is thus possible, in some instances, that stopping distance may be sacrificed due to insufficient braking of the high wheel in case of changing characteristics of the road surface.
Under certain circumstances, however, it is also possible that the braking pressure difference (.DELTA.P) may be too high for critical vehicles, so that they can be controlled only with difficulty or not at all by the driver.
The invention has as its object to provide a process for reducing the yawing moment in a vehicle equipped with an anti-lock brake system (ABS), relieving the driver on the one hand from excessive counter-steering, and on the other hand shortening the stopping distance by comparison with the known systems.